ABSTRACT Systematic exploitation of heterosis via breeding of F1 hybrids is a major contributor to genetic gain in many major crops, including sorghum ( Sorghum bicolor L. Moench). Different cytoplasmic male sterility (CMS) systems have been identified for controlled F1 hybrid seed production in sorghum, with the so‐called A1 and A2 CMS systems being predominantly deployed. However, due to genetic differences in cytoplasmatically determined traits, significant effects of the cytoplasm carrying different CMS systems on agronomic performance or biotic and abiotic stress tolerance have been described. Cold tolerance is an essential trait for adaptation of sorghum at high altitudes or in temperate areas at high latitudes. Here, we investigated the effects of cytoplasm on juvenile and reproductive cold stress by comparing iso‐nuclear crosses with CMS female lines carrying A1 and A2 cytoplasm, respectively. During juvenile cold stress, 14 hybrids were tested under climate chamber conditions. Six hybrids were tested during reproductive cold stress in four field trials and one climate chamber experiment. Although significant hybrid × cytoplasm interaction was observed, one‐way ANOVA comparing A1 versus A2 iso‐nuclear hybrids for juvenile cold tolerance traits revealed no significant differences. However, at the reproductive stage, individual differences were observed between A1 and A2 cytoplasm with regard to photosynthetic performance, stomatal conductance and pollen fertility during certain developmental stages, although these had no effect on yield performance. During reproductive cold stress under field conditions, the A1 cytoplasm was superior to A2 cytoplasm in all investigated yield traits (panicle harvest index, seed weight and number, thousand kernel weight). Presently, the A1 cytoplasm is used almost exclusively in global sorghum hybrid seed production. Implementation of additional cytoplasmic systems could help to increase diversity in sorghum breeding programmes and provide a back‐up against possible disease outbreaks associated with a certain cytoplasm, as occurred in maize. We demonstrate that the A2 cytoplasm can be a viable alternative for temperate sorghum breeding, at least with regard to juvenile cold tolerance. The results of this study will help to expand breeding programmes and increase the diversity in temperate sorghum breeding by implementing an alternative cytoplasm.
Kravcov et al. (Thu,) studied this question.